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2018 Will Be The Year Humanity Directly 'Sees' Our First Black Hole

Black holes
are some of the most incredible objects in the Universe. There are places where
so much mass has gathered in such a tiny volume that the individual matter
particles cannot remain as they normally are, and instead collapse down to a
singularity. Surrounding this singularity is a sphere-like region known as the
event horizon, from inside which nothing can escape, even if it moves at the
Universe's maximum speed: the speed of light.

While we
know three separate ways to form black holes, and have discovered evidence for
thousands of them, we've never imaged one directly. Despite all that we've
discovered, we've never seen a black hole's event horizon, or even confirmed
that they truly had one. Next year, that's all about to change, as the first
results from the Event Horizon Telescope will be revealed, answering one of the
longest-standing questions in astrophysics.

The
locations of the radio dishes that are planned be part of the Event Horizon
Telescope array.

The idea of a
black hole is nothing new, as scientists have realized for centuries that as
you gather more mass into a given volume, you have to move at faster and faster
speeds to escape from the gravitational well that it creates. Since there's a
maximum speed that any signal can travel at — the speed of light — you'll reach
a point where anything from inside that region is trapped. The matter inside
will try to support itself against gravitational collapse, but any
force-carrying particles it attempts to emit get bent towards the central
singularity; there is no way to exert an outward push. As a result, a
singularity is inevitable, surrounded by an event horizon. Anything that falls
into the event horizon? Also trapped; from inside the event horizon, all paths
lead towards the central singularity.

An
illustration of an active black hole, one that accretes matter and accelerates
a portion of it outwards in two perpendicular jets, may describe the black hole
at the center of our galaxy in many regards.

Practically,
there are three mechanisms that we know of for creating real, astrophysical
black holes.

1. When a
massive enough star burns through its fuel and goes supernova, the central core
can implode, converting a substantial fragment of the pre-supernova star into a
black hole.

2. When two
neutron stars merge, if their combined post-merger mass is more than about
2.5-to-2.75 solar masses, it will result in the production of a black hole.

3. And if
either a massive star or a cloud of gas can undergo direct collapse, it, too,
will produce a black hole, where 100% of the initial mass goes into the final
black hole.

Artwork
illustrating a simple black circle, perhaps with a ring around it, is an
oversimplified picture of what an event horizon looks like.

Over time,
black holes can continue to devour matter, growing in both mass and size
commensurately. If you double the mass of your black hole, its radius doubles
as well. If you increase it tenfold, the radius goes up by a factor of ten,
also. This means that as you go up in mass — as your black hole grows — its
event horizon gets larger and larger. Since nothing can escape from it, the
event horizon should appear as a black "hole" in space, blocking the
light from all objects behind it, compounded by the gravitational bending of
light due to the predictions of General Relativity. All told, we expect the
event horizon to appear, from our point of view, 250% as large as the mass
predictions would imply.

A black hole
isn't just a mass superimposed over an isolated background, but will exhibit
gravitational effects that stretch, magnify and distort background light due to
gravitational lensing.

Taking all
of this into account, we can look at all the known black holes, including their
masses and how far away they are, and compute which one should appear the
largest from Earth. The winner? Sagittarius A*, the black hole at the center of
our galaxy. Its combined properties of being "only" 27,000 light
years distant while still reaching a spectacularly large mass that's 4,000,000
times that of the Sun makes it #1. Interestingly, the black hole that hits #2
is the central black hole of M87: the largest galaxy in the Virgo cluster.
Although it's over 6 billion solar masses, it lies some 50-60 million light
years away. If you want to see an event horizon, our own galactic center is the
place to look.

Some of the
possible profile signals of the black hole's event horizon as simulations of
the Event Horizon Telescope indicate.

If you had a
telescope the size of Earth, and nothing in between us and the black hole to
block the light, you'd be able to see it, no problem. Some wavelengths are
relatively transparent to the intervening galactic matter, so if you look at
long-wavelength light, like radio waves, you could potentially see the event
horizon itself. Now, we don't have a telescope the size of Earth, but we do
have an array of radio telescopes all across the globe, and the techniques of
combining this data to produce a single image. The Event Horizon Telescope brings
the best of our current technology together, and should enable us to see our
very first black hole.

A view of
the different telescopes contributing to the Event Horizon Telescope's imaging
capabilities from one of Earth's hemispheres. Data was taken in April that
should enable the detection (or non-detection) of an event horizon around
Sagittarius A* within the next year.

Instead of a
single telescope, 15-to-20 radio telescopes are arrayed across the globe,
observing the same target simultaneously. With up to 12,000 kilometers
separating the most distant telescopes, objects as small as 15 microarcseconds
(μas) can be resolved: the size of a fly on the Moon. Given the mass and
distance of Sagittarius A*, we expect that to appear more than twice as large
as that figure: 37 μas. At radio frequencies, we should see lots of charged
particles accelerated by the black hole, but there should be a "void"
where the event horizon itself lies. If we can combine the data correctly, we
should be able to construct a picture of a black hole for the very first time.

Five
different simulations in general relativity, using a magnetohydrodynamic model
of the black hole's accretion disk, and how the radio signal will look as a
result. Note the clear signature of the event horizon in all the expected
results.

The telescopes
comprising the Event Horizon Telescope took their very first shot at observing
Sagittarius A* simultaneously last year. The data has been brought together,
and it's presently being prepared and analyzed. If everything operates as
designed, we'll have our first image in 2018. Will it appear as General
Relativity predicts? There are some incredible things to test:

1. Whether
the black hole has the right size as predicted by general relativity,

The
orientation of the accretion disk as either face-on (left two panels) or
edge-on (right two panels) can vastly alter how the black hole appears to us.

Whatever we
do (or don't) wind up discovering, we're poised to make an incredible
breakthrough simply by constructing our first-ever image of a black hole. No
longer will we need to rely on simulations or artist's conceptions; we'll have
our very first actual, data-based picture to work with. If it's successful, it
paves the way for even longer baseline studies; with an array of radio
telescopes in space, we could extend our reach from a single black hole to many
hundreds of them. If 2016 was the year of the gravitational wave and 2017 was
the year of the neutron star merger, then 2018 is set up to be the year of the
event horizon. For any fan of astrophysics, black holes, and General Relativity,
we're living in the golden age. What was once deemed "untestable" has
suddenly become real.

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